Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere

Abstract

[1] In the context of recent observations, microphysical models, and laboratory data, a photochemical model of Titan's atmosphere, including updated chemistry focusing on rate coefficients and cross sections measured under appropriate conditions, has been developed to increase understanding of these processes and improve upon previous Titan photochemical models. The model employs a two-stream discrete ordinates method to characterize the transfer of solar radiation, and the effects of electron-impact, cosmic-ray deposition, and aerosol opacities from fractal and Mie particles are analyzed. Sensitivity studies demonstrate that an eddy diffusion profile with a homopause level of 850 km and a methane stratospheric mole fraction of 2.2% provides the best fit of stratospheric and upper atmosphere observations and an improved fit over previous Titan photochemical models. Lack of fits for C 3 H 8 , HC 3 N, and possibly C 2 H 3 CN can be resolved with adjustments in aerosol opacity. The model presents a benzene profile consistent with its detection in Titan's stratosphere , which may play an important role in the formation of Titan hazes. An electron peak concentration of 4200 cm À3 is calculated, which exceeds observations by 20%, considerably lower than previous ionosphere models. With adjustments in aerosol opacities and surface fluxes the model illustrates that reasonable fits to existing observations are possible with a single eddy diffusion profile, contrary to the conclusions of previous Titan models. These results will aid in the receipt and interpretation of data from Cassini-Huygens, which will arrive at Titan in 2004 and deploy a probe into Titan's atmosphere in January 2005.

Titan’s Carbon Budget and the Case of the Missing Ethane

“…This process acts as a major source of free hydrogen atoms, which play a prominent role in hydrocarbon growth through pressure-induced H-addition chain reactions in Titan's stratosphere. 16 Other condensates and haze account for 38% of methane loss, with 22% due to escape. These processes would result in a liquid ethane layer of 400 m on Titan's surface over geologic time, but this layer could be reduced to levels indicated today by episodic outgassing of methane, thermodynamics, and the chemical processing of surface material.…”

“…Methane absorbs ultraviolet photons shortward of 1450 Å, with Lyman R photons accounting for 75% of methane photoabsorption. 16 The resulting photodissociation takes place in Titan's upper atmosphere, peaking at about 800 km, the altitude of Lyman R deposition, at a rate of 2.5 × 10 9 molecules cm -2 s -1 , as obtained by integrating the curve in Figure 1, referred to the surface.…”

“…Furthermore, GCR can play a substantial role in the formation of certain nitriles. 16 Observations of these nitriles in the stratosphere by CIRS and on the surface by GC/MS can provide a proxy for the penetration of galactic cosmic rays into the lower atmosphere and their influence on surface material. Material released from the interior in cryovolcanic processes, especially water, ammonia, and other volatiles from Titan's interior ocean, may also chemically interact with the surface liquids and modify them.…”

Between heaven and Earth: the exploration of Titan

“…Trace amounts of several photochemically produced hydrocarbons and nitriles are present in the upper atmosphere (e.g. Raulin, Wilson and Atreya 6 and Waite et al 7 ), where 50 ppb of CO 2 is created by reactions between CO and infalling H 2 O (Samuelson et al 8 ). Thus, we find a world literally frozen in time, where we can study chemical and physical processes that may have been important during our planet's earliest history.…”

Composition and chemistry of Titan's stratosphere

“…for the most recent ones, Wilson & Atreya 2004;Lebonnois 2005;Lavvas et al 2008a,b;Vuitton et al 2008). These models incorporate chemical sinks and losses (photodissociation, 2-and 3-body reactions) and vertical transport parametrized by an altitude-varying eddy mixing coefficient.…”